<template>
  <ul class="list">
    <li class="item" 
      v-for="(item, key) in cities" 
      :key="key" 
      :ref="key"
      @click="handleLetterClick"
      @touchstart.prevent="handleTouchStart"
      @touchmove="handleTouchMove"
      @touchend="handleTouchEnd"
    >
      {{key}}
    </li>
  </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props:{
    cities:Object
  },
  computed: {
    letters (){
      const letters= [];
      for(let i in this.cities){
        letters.push(i)
      }
      return letters
    }
  },
  data (){
    return {
      touchStatus: false,
      startY:0,
      timer:null
    }
  },
  updated (){
    this.startY=this.$refs['A'][0].offsetTop
  },
  methods:{
    handleLetterClick:function(e){
      // console.log(e.target.innerText)
      this.$emit('change',e.target.innerText)
    },
    handleTouchStart (){
      this.touchStatus=true
    },
    handleTouchMove (e){
      if(this.touchStatus){
        if(this.timer){
          clearTimeout(this.timer)
        }
        //函数节流
        this.timer=setTimeout(()=>{
          const touchY=e.touches[0].clientY-79
          const index=Math.floor((touchY-this.startY)/20)
          if(index >=0 &&index<this.letters.length){
            this.$emit('change',this.letters[index])
          } 
        },16)
      }
    },
    handleTouchEnd (){
      this.touchStatus=false
    }
  }
}
</script>

<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
  .list
    display: flex
    flex-direction: column
    justify-content: center
    position: absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      line-height: .4rem
      text-align: center
      color: $bgColor
</style>
